The present invention relates generally to the field of industrial process emission control, and specifically to a photochemical system and method for removing PCDD-PCDF compounds from the gaseous discharge of various industrial processes.
A wide variety of industrial processes generate, as a byproduct, an output or xe2x80x9ceffluentxe2x80x9d in a gaseous form. Many such effluent streams contain chemicals that are known environmental hazards, are toxic to humans and animals, or both. Economically effective removal of these hazardous materials from industrial process effluent streams is desirable, and additionally is often mandated according to quantified levels by various regulatory agencies.
Particularly hazardous toxic organic compounds are known to exist in low concentrations in the effluent gas streams of a number of industrial processes, such as municipal waste incinerators, medical waste incinerators, and in portland cement plants. These toxic organic compounds are known as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, more commonly known as dioxins and furans or xe2x80x9cPCDD-PCDF compounds.xe2x80x9d These PCDD-PCDF compounds comprise a family of some 210 similar organic congeners that differ primarily with respect to the extent and position of chlorine substitutions on their aromatic rings. The congeners differ significantly in their toxicity to humans. The congener 2, 3, 7, 8-tetrachforodibenzo-p-dioxin (2, 3, 7, 8 PCDD) is the most toxic of this family, with the congeners having penta- through octa-substituted chlorine atoms exhibiting a lower level of toxicity, and the remaining congeners still less toxic. The PCDD-PCDF compounds are ranked in their toxicity relative to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, that is assigned a Toxicity Equivalent Factor (TEF) of 1.0. After many years of research, researchers have determined the relative toxicities for the 210 different compounds that make up the PCDDs and PCDFs with the most toxic 2, 3, 7, 8-tetrachlorodibenzodioxin being assigned a toxic equivalency factor (TEF) of 1.0. The others are assigned values of 0.5, 0.1, 0.05, 0.01, 0.001 or 0.0. To calculate the toxic equivalent quotient, (TEQ), emitted from the source, the measured quantity of each of these compounds is multiplied by its assigned TEF. Then the results are summed to produce a single TEQ value. It is this TEQ value that is used in calculating the health risks associated with the Emission of PCDD and PCDF compounds.
Industrial process effluent streams are regulated with reference to their total TEQ values. Due to the significant toxicity of the PCDD-PCDF compounds, their emissions are strictly limited by the U.S. EPA to Maximum Achievable Control Technology (MACT) standards. Specifically, the regulatory standards for the gaseous effluent from portland cement production facilities is 0.2 nanograms TEQ per cubic meter corrected to 7% oxygen. Typical portland cement pyroprocessing production processes may exhibit a PCDD-PCDF concentration of approximately 0.001 to 5.0 nanograms TEQ per meter cubed corrected to 7.0% oxygen. A reduction of PCDD-PCDF compound concentrations in portland cement pyroprocessing system effluent streams is required for sources emitting more than the allowed limits.
Various approaches to reducing PCDD-PCDF compounds from portland cement pyroprocessing effluent streams are known in the art. For example, it is known to select the raw materials used to prepare the raw feed (primarily comprising limestone, clay, and sand, which furnish the necessary elements calcium, silicon, aluminum, and iron) to have minimum concentrations of organic material. Additionally, it is believed that PCDD-PCDF compound formation from organic compounds is related to gas stream temperature. Consequently, the gaseous effluent stream has been directed through fluid mists and washes, utilizing evaporative cooling to reduce the gas temperatures to below 400xc2x0 F. However, portland cement plants may operate in an arid environment, wherein water is scarce and expensive, and thus evaporative cooling temperature control is not cost effective. Furthermore, there are cases in which temperature reduction is not sufficient for the suppression of PCDD-PCDF compounds. There are also cases where gas temperature reduction impairs the operation of particulate matter control systems and thereby causes increased emissions of hazardous metals contained in the particulate matter. Hence, there remains a need in the art for a system and method of removing hazardous levels of PCDD-PCDF compounds from industrial process effluent gas streams that can operate at temperatures above 400xc2x0 F. with little or no evaporative cooling.
The present invention entails a process of detoxifying PCDD or PCDF compounds within an industrial gas stream. The basic process entails irradiating the industrial gas stream so as to detoxify the PCDD or PCDF compounds therein. Basically, the process of the present invention acts in one of two ways. First, the process entails irradiating the PCDD or PCDF compounds entrained within the gas stream so as to convert the PCDD or PCDF compounds to less toxic compounds. Secondly, the process may irradiate PCDD and/or PCDF precursors and in the process chemically modify those precursor compounds so as to produce different chemical compounds that do not participate in chemical reactions that result in the formation of PCDD and PCDF compounds.
In one particular embodiment of the present invention, the gas stream is treated by removing particulate matter from the gas stream. The treated gas stream is then subjected to irradiation within the spectral range of 290-500 nanometers which results in the production of at least one free radical hydrogen donor that reacts with the PCDD or PCDF compound to yield a lower chlorinated form PCDD or PCDF compound. In this embodiment, the gas stream may be irradiated with light within the spectral range of 250-500 nanometers prior to treatment for the purpose of chemically modifying PCDD or PCDF precursor compounds.
The present photochemical process is applicable to a number of industrial processes, such as waste incinerators and cement plants. In the case of cement plants, the present invention entails irradiating the effluent gas stream that passes from the pyroprocessing system of the cement plant. In this regard, the gas stream is irradiated with light falling within the spectral range of 290-500 nanometers. Preferably in a preferred embodiment, this radiation takes place after the gas stream has been treated by removing particulate matter therefrom. By irradiating the gas stream after treatment, free radical hydrogen donors are generated in the process and the free radical hydrogen donors react with the PCDD or PCDF compounds within the gas stream to yield a less toxic and lower chlorinated PCDD or PCDF compound forms. Further, in this process, additional stages of radiation may be employed. In this regard, prior to removing the particulate matter, the gas stream may be irradiated with light that falls within the spectral range of 250-500 nanometers for the purpose of reacting with and chemically modifying PCDD or PCDF precursor compounds. This irradiation should yield different chemical compounds that do not participate in chemical reactions that result in the formation of PCDD-PCDF compounds.
Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention.